Field effect electroacoustic transducer



April 1, 1969 c. w. REEDYK FIELD EFFECT ELECTROACOUSTIC TRANSDUCER Filed Jan. 17, 1966 V3 DISTANCE United States Patent 3,436,492 FIELD EFFECT ELECTROACOUSTIC TRANSDUCER Cornelis W. Reedyk, Ottawa, Ontario, Canada, assignor to Northern Electric Company Limited, Montreal,

Quebec, Canada Filed Jan. 17, 1966, Ser. No. 521,029 Int. Cl. H041 19/00 US. Cl. 179-111 11 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an electroacoustic transducer having desirable electrical and acoustical properties. The transducer of the present invention consists of a device resembling a large field effect transistor having a movable gate electrode which is used as the diaphragm and one plate of a capacitor type microphone. The dielectric film separating the gate from the remainder of the transistor may in accordance with a preferred form of the invention be formed of an electret with the gate being a metallized layer on the electret. Alternatively, the dielectric film separating the gate from the remainder of the tranformed as a metallic film adhered to the side of the insulating film remote from the semiconductor body. In this case an external source of bias voltage is connected between the gate and the semiconductor body. In either case the electret or insulating film will be stretched across the semiconductor in intimate engagement therewith. Electric field variations in the semiconductor caused by movement of the movable gate electrode due to impinging acoustic waves cause changes in the resistance of the semiconductor in a known manner and thus cause modulation of the current flowing through the semiconductor to produce an electrical signal analogous to the acoustic wave. Accordingly, the transducer of the present invention may be termed an integral field effect transducer.

It is an object of the present invention to provide a new and improved field effect transducer which may be used as a microphone.

In accordance with the invention there is provided an acoustic pressure transducer comprising a dielectric film having a thin metallic layer applied to one surface thereof, the other surface of said film overlying the surface of a semiconductive body having a pair of terminals for connection to an external circuit, whereby acoustic waves impinging on the thin metallic layer will modulate current flowing between said pair of terminals.

It will be appreciated by those skilled in the art that microscopic air spaces occur between the surface of the film and the surface of the semiconductor since the film must move toward the semiconductor body an extremely small but finite distance under the influence of an impinging acoustic wave.

In accordance with a preferred form of the invention, the dielectric film is formed as an electret having a thin metallic layer deposited on one face. The formation of the electret is conventional and may for example be'carried out by exposing a thin sheet of plastic material such as polyethylene terephthalate to an electrostatic field having a potential gradient of say 20 kv. per cm. and at the same time heating the sheet to C. The sheet is allowed to cool slowly in the field and retains its polarization after removal from the field, thus forming an electret.

In drawings which illustrate the structure and operation of embodiments of the present invention:

FIGURE 1 is a cross-section through a typical field effect transducer constructed in accordance with one aspect of the present invention,

FIGURE 2 is an alternative embodiment in accordance with the present invention,

FIGURE 3 is a section partly in perspective illustrating the structure of one embodiment of a field effect transducer constructed in accordance with the invention,

FIGURE 4 is a schematic diagram illustrating the use of a transducer in accordance with the invention,

FIGURE 5(a) is a diagram of potential distribution in a rectangular strip in the semiconductor of the field effect transducer,

FIGURE 5 (b) is a diagram illustrating the potential distribution in a semiconductor of disc shape, and

FIGURE 6 is a cross section of a telephone microphone similar to FIGURE 3.

Referring to FIGURE 1, there is shown one embodiment of a transducer in accordance with the present invention consisting of a thin film metallic layer 10 on an electret substrate 11 which may be formed of metallized polyethylene terephthalate foil (such as that solid under the registered trademark Mylar), or any other suitable material, such as polytetrafluoroethylene sold under the registered trademark Teflon, in accordance with known construction techniques. The electret 11 is placed against the semiconductor body 12 having electrodes 13 and 14. The electrodes 13 and 14 are connected in series with a battery 15 and a load resistance 16. A connection is also made from the metallic layer 10 to the electrode 13.

Acoustic pressures P applied to the upper face of the metallic layer 10 will cause motion of the electret 11 and variation in the electric field in semiconductor 12. This varying field, will modulate the current flowing between the terminals 13 and 14 in the semiconductor 12. Thus, the fluctuating voltage appearing across the load resistance 16 will be an electric analogue of the acoustic wave impinging on the layer 10. It will be appreciated that since the transducer of the present invention is a field effect device that electrical outputs may be obtained which are readily coupled to associated circuitry of the transmission of signals.

In accordance with current practice the load resistance selected for operation with a transducer of the invention would most often be lower than the drain-to-source resistance so that the transducer acts as a current source. To match to normal microphone impedances, additional circuitry would normally be incorporated with the transducer. The output level of the device will, of course, depend on the voltage supply and the load resistance. However the transducer of the present invention provides an appreciably greater output than most microphone cartridges are capable of and definitely much larger than normal capacitor microphones. The output of the transducer of the present invention would not normally be of the same order as say a carbon microphone, at the present state of the semiconductor art.

FIGURE 2 illustrates a transducer similar to the transducer of FIGURE 1 with the exception that the electret 11 has been replaced by an insulating film and the metallic layer 30 is now connected to the electrode 13 via the battery 31 so that a bias voltage is applied to the layer 30 which as before acts as the gate electrode of the field effect device. The polarity of the battery 31 will depend upon the type of material in the semiconductor body 12 and this polarity will be conventional as for other field effect transistors.

Thus in accordance with the invention either an electret 11 or a dielectric film 11 together with a bias battery 31 may be used to obtain the necessary gate field for the field effect transducer, and for both embodiments as illustrated in FIGURES 1 and 2 acoustic pressure Waves P impinging upon the metallic layer or 30 cause motion of the electret 11 or the film 11 thus varying the flow of current through the semiconductor 12 to provide an electrical output which is the analogue of the acoustic input.

FIGURE 3 illustrates a practical transducer constructed in accordance with the present invention in form for use as a telephone microphone. As illustrated, a semi-conductor film 12 is deposited as a thin film layer on a substrate of insulating material -17. The semiconductor film 12 and the substrate 17 are perforated at 18 to couple into a cavity 19. The electret 11 with its metallized outer surface .10 is clamped between the back plate 17 and a metal case 20 which also makes contact with a metallized layer 10 through the upper fiange 21. The metal case 20 has the advantage that it provides electrical connection to the components of the transducer and at the same time acts as an electrostatic shield. The battery and load (not shown in FIGURE 3) are connected in series with the metal case 20, the semiconducting layer 112 and a terminal 22 provided centrally of the metallic case 20. Spacers 23 and 24 separate the back plate 17, from the end 25 of the metallic case 20. An insulated bushing 26 prevents a short circuit between the central terminal 22 and the metallic case 20.

If the output of the transducer shown in FIGURE 3 should prove insufiicient in practice to obtain a sufficiently large signal, then further amplifiers may be connected to the transducer to provide a desired output amplitude.

Due to the finite conductivity of the semiconductor body 12 in FIGURE 3, a potential gradient will exist along the path from the case 20 to the central terminal 22. The portion of the semiconductor body 12. in contact with the case 20 will be referred to as a cathode and the portion of the semiconductor body 12 in contact with the central terminal 22 will be referred to as the anode. The potential gradient from the cathode to the anode will tend to reduce the area of the semiconductor that is influenced by the varying field strength between surface of the electret 11 and the surface of the semiconductive body 12. In the case of a rectangular strip with electrodes 13 and 14, as shown in FIGURE 1, this potential gradient will be constant if the semiconductive body 12 is homogeneous, i.e., the conductivity is constant. In the case of a disc as shown in FIGURE 2 with the electrode 14 effectively in the centre and the electrode 13 at the rim the potential gradient will not be constant due to the increase in current density toward the centre; again assuming the bulk conductivity is constant. This effect may be used to advantage when electrodes 13 and 14 are poled in such a manner that the electrostatic field due to polarizationof the electret and the potential distribution on the semiconductor will be aiding over the largest possible area.

FIGURE 4 illustrates schematically a transducer of the, present invention coupled to a following amplifier, symbolically shown as a single transistor. As before the transducer may consist of a metallic layer 10, an electret .11 on which the metallic layer is applied, *and a semiconductive body 12 with electrodes 13 and 14 attached thereto. The metallic layer 10 is connected to the electrode 13, and the electrode 14 is connected to the base of transistor 27 in the emitter circuit of which the load resistance 16 is located. As before, a battery 15 is provided to energize the circuit and the output from the transducer and its associated transistor amplifier 27 is obtained across the load resistance 16.

FIGURE 5(a) illustrates the voltage distribution across a rectangular strip x12 having terminals 13 and 14. As

4 can be seen the potential gradient is linear along the length of the strip .12 from terminal .13 to terminal 14.

FIGURE 5(b) illustrates the potential distribution in a semiconductor '12 formed as a disc with the terminal 13 being an annular conductor attached to the periphery of the disc 12 and with the terminal 14 being at the centre of the disc. As shown the potential distribution is nonlinear due to the increasing current density toward the central terminal 14.

FIGURE 6 is a cross-section of a transducer constructed in accordance with the present invention which is enclosed in a mechanical structure making it suitable for use as a microphone in a telephone hand-set. A metal case 32 which is in the form of a hollow cylindrical member, is provided with a central insulated bushing 33 through which a central terminal 34 of the microphone assembly projects for connection to the remainder of the telephone circuit. An annular terminal 35 is similarly provided for the remaining contact to the external circuit. The upper portion of the metal case 32 is provided with a cap 36 which serves to retain the dielectric film 37 in position in relation to the remaining portions of the transducer. The upper surface of the film 37 is provided with a metallized layer 38 and the central portion 39 of the dielectric film 37 has been electrized as hereinbefore described.

Beneath the electrized portion 39 of the film 37, is a small gap 40, and beneath this gap the polycrystalline semiconducting film 41 is located. The semiconductor film 41 is provided with a source or cathode 42 which is of annular configuration and a drain or anode 43' located centrally of the semiconducting film 41. The film is formed on a substrate 44 which is held in position by a porous or perforated metallic back plate 45 which provides acoustical coupling into the cavity 46 and electrical contact with the cathode or source electrode 42 and the metallic case 32 and hence the annular terminal 35.

As before acoustic pressure waves impinging on the metallized surface 38 cause variations in the electric field in the semiconductor 41 and hence variations in the electric current flowing between the terminals 34 and 35 of the transducer. This variation in current is analogous to the variation in the acoustic pressure and thus an electrical output is obtained which is the electrical analogue of the acoustic input.

In addition, it may be possible by using thin film semiconductor techniques to grade the conductivity in a predetermined manner to obtain a desired potential gradient and optimum operation of the transducer. Alternatively, various patterns for the electrodes 13 and 14 may be used to shape the potential distribution along the surface of the semi-conductor 12 or to distribute the current more evenly according to well known semiconductor techniques.

It is considered that the present invention will have its major application in the field of wire telephony as a replacement for the carbon transmitter and further, it is considered that thin film or integrated circuit techniques are most suitable for construction of the transducer of the present invention.

I claim:

1. An electroacoustic transducer comprising a semiconductor body having a source and a drain, a flexible insulating film overlying a surface of said semiconductor body, a metallized layer applied to said insulating film and adapted for connection to a source of bias voltage to bias said metallized layer in relation to said semiconductor body, said source and drain of said semiconductor body being adapted for connection to an external source of operating voltage and an external load resistance whereby acoustic pressure applied to said metallized film causes said insulating film to move with respect to said semiconductor body, thus varying the electrical filed in said semiconductor body and causing variations in the voltage drop across said external load resistance in accordance with the acoustic input to said transducer.

2. A transducer as claimed in claim 1 wherein said semiconductor comprises an elongated strip, and said source and said drain are located at opposite ends of said strip.

3. A transducer as claimed in claim 1 wherein said semiconductor is formed as a circular disc, one of said source and said drain being located in the centre of said disc and the other of said source and said drain being located at the periphery of said disc.

4. An acoustic pressure transducer comprising an electret having a thin metallic layer applied to one surface thereof, the other surface of said electret being in intimate engagement with the surfaces of a semiconductive body having a pair of terminals for connection to an external circuit, whereby acoustic waves impinging on the thin metallic layer will modulate current flowing between said pair of terminals.

5. A transducer as claimed in claim 4 wherein said semiconductor comprises an elongated strip, and said terminals are positioned at opposite ends of said elongated strip.

6. A transducer as claimed in claim 4 wherein said semiconductor is formed as a circular disc, one of said terminals being fixed to said disc in the centre thereof, the other terminal being fixed to the periphery of said disc.

7. An electroacoustic transducing circuit comprising a transducer as claimed in claim 4 in combination with a source of electric potential and a load resistance con nected in circuit with said terminals, whereby a sound pressure wave inpinging on said thin metallic layer will cause variation in the current flowing through said semiconductor body and a voltage will appear across said load resistance which is the electrical analogue of the acoustic input to said transducer circuit.

8. A transducing circuit as claimed in claim 7, wherein said semiconductor comprises an elongated strip, and said terminals are positioned at the opposite ends of said elongated strip.

9. A transducing circuit as claimed in claim 7, wherein said semiconductor is formed as a circular disc, one of said terminals being fixed to said disc in the centre thereof, the other terminal 'being fixed to the periphery of said disc.

10. A telephone microphone comprising a substantially cylindrical metallic case having one open end and having a central insulated terminal and an outer annular terminal concentric with said central terminal on the .closed end, means in said metallic case supporting a semiconductive body having a pair of electrodes, one electrode of said semiconductive body being connected to the central terminal, and the other electrode being connected to the annular terminal, a thin insulated film having a metallized surface which is exposed to acoustic pressure waves, said insulating film being an electret and positioned adjacent to and in intimate contact with the semiconductive body whereby acoustic pressure waves an said metallized film cause variations in the electric field in said semiconductive body and an electric current flowing through said semiconductive body from an external source is modulated by said varying field to produce an electrical analogue of the acoustic input.

11. A telephone microphone as claimed in claim 10 wherein said metal case supports said semiconductive body in spaced relation from the closed end thereof, thereby providing an acoustic cavity behind said transducer to improve the acoustic efficiency of said transducer.

References Cited UNITED STATES PATENTS 3,016,752 1/1962 Huebschmann 317235 3,287,506 11/1966 Hahnlein 1791l0 KATHLEEN H. CLAFFY, Primary Examiner. A. A. MCGILL, Assistant Examiner.

U.S. Cl. X.R. 179-421; 317235 

